1. Technical Field
The present embodiments relate generally to the field of railroad couplers, and more specifically, to the casting of railcar coupler knuckles using subsurface chills to reduce micro-shrinkage in a high-stress area of the casting.
2. Related Art
Railcar couplers are disposed at each end of a railway car to enable joining one end of such railway car to an adjacently disposed end of another railway car. The engageable portion of each of these couplers is known in the railway art as a knuckle.
Typically, a knuckle is manufactured by a mold—usually made of sand—and several cores that are disposed within the mold. The mold shapes the outside of a casting. The cores are disposed to shape the inside or outside of a casting. Without the internal cores, the casting would be made of solid metal. The outside cores help shape the exterior of the casting. The internal cores commonly are referred to as a finger core in the front portion of the knuckle, a pivot pin core in the center of the knuckle, and a kidney core at the rear of a knuckle, and form the cavities in the knuckle upon casting.
During the casting process itself, the interrelationship of the mold and the internal cores make the difference in producing a satisfactory railway coupler knuckle. Many knuckles fail from internal and/or external inconsistencies in the metal throughout the thickness of the knuckle. If one or more cores move during the casting process, then some knuckle walls may end up thinner than others, resulting in offset loading and, in turn, in an increased failure risk during use of the knuckle.
The external features of a coupler knuckle should meet railroad industry standards both because of initial acceptance of the knuckle and for its successful performance in service. External features of a knuckle (7 in FIG. 3) that must be formed properly for successful knuckle performance in service include a pulling face contour (30 in FIG. 3) and a throat (42 in FIG. 3). The pulling faces of mating couplers contact each other when freight cars are coupled together and transmit the forces pulling the train. These pulling forces can be substantial. Moments of force from the pulling face converge on the throat, a part of the knuckle that often fails because of the amount of force and the thinning of the throat area between the surface and a C-10 pin hole (38 in FIG. 3). For this reason, railroad industry standards exist that specify the shape of the pulling face contour and recommended practices for forming the coupler. Inconsistent or out of tolerance pulling face contours can result in poor coupling/uncoupling performance of the coupler or in detrimental load paths for the pulling load. One patent that discusses the importance of the proper performance of the pulling face is U.S. Pat. No. 7,337,826 entitled “Railway Car Coupler Knuckle Having Improved Bearing Surface” (the '826 Patent). The '826 Patent describes techniques for casting a knuckle coupler with an enhanced bearing surface. The '826 Patent, however, does not address the imperfections that can form on or below the knuckle surface during casting.
Coupler knuckles are generally manufactured from cast steel or alloys. By way of example, when a molten metal is introduced into a mold during casting, it is prone to shrinking as it cools and solidifies. This is known as “shrinkage” or “micro-shrinkage” and occurs because most metals are less dense as a liquid than as a solid. Shrinkage may occur on the outside of the casting, the inside of the casting, or both. Shrinkage may lead to the knuckle forming shrinkage defects and/or solidification related defects, and/or even the formation of a void in certain portions of the knuckle. This could cause premature wear on the coupler to or result in premature fatigue and/or failure.
One technique used to overcome micro-shrinkage is the inclusion of risers (255 in FIG. 4) in the mold. The risers feed the volumes of the casting that are prone to shrinkage with additional casting material as the casting cools. However, once the knuckle is cast, the risers must be removed, typically by surface grinding. This may cause damage to the knuckle's surface and cause the knuckle to prematurely fatigue and/or fail. Moreover, risers and/or large ingates (256 in FIG. 4), e.g., material that connects the risers to the casting, are limited by location in their ability to provide for a uniform thickness throughout the casting, maintain precise part profile, and they lose their effectiveness in areas farther away from the riser. Other benefits and drawbacks of using riser systems are discussed in the '967 Application.
Internal and external metal chills have also been used to help remove heat from the poured metal in the location of the chill in order to promote and direct solidification and limit the amount of shrinkage in the vicinity of the small area in which they are located. Sometimes chills can alleviate the need to have as many risers or have ingates located as close to each other. However, there are some disadvantages relating to the use of chills including additional costs. Furthermore, the chills must usually be made of the same material as the casting and sometimes fail to fuse with the casting, or must be removed from the cast knuckle later. External chills become attached to the knuckle surface and require removal followed by extra finishing steps that not only increase costs but can leave scars or defects on the surface of the knuckle casting. Use of chills takes much experimentation, and therefore failure, before finding a solution with improved results that justify the added cost and/or casting defects in certain parts of the knuckle casting. What is needed, therefore, is an improved chill and deployment thereof to obtain the benefits of using chills without the above-listed disadvantages.